1. Field of the Invention
The present invention relates to a rotary hearth furnace for producing reduced metal through heating and reducing carbon containing materials composed of at least metal oxide-containing material and carbonaceous reduction material, and in particular, to a structure of a hearth.
2. Description of Related Art
Lately, as steel products are actively produced by using an electric arc furnace, demand for reduced iron is gradually increasing due to the problems for supply of scrap as the main raw material of the steel products or request for producing high quality steel in the electric arc furnace.
As one of the processes for producing reduced iron, a method is well known, by which fine ore and coal material such as powder coal or coke are mixed to form a agglomerative material such as pellet, which is charged into a rotary hearth furnace and heated up to a high temperature to reduce iron oxide in iron ore for obtaining solid metallic iron.
Further, such a reduction method can be used not only for production of metallic iron but also for reduction of nonferrous metal such as Ni and Cr.
Reduced iron is produced by the rotary hearth furnace as an example as follows (refer to FIG. 5).
(1) Powdered iron oxide and powdered carbonaceous material are mixed and pelletized to form green pellets.
(2) The green pellets are heated up to such a temperature area that combustible volatile components generated from the pellets may not ignite to remove contained moisture to obtain dry pellets (hereinafter will be simply referred to as pellet).
(3) The pellets 7 are supplied into the rotary hearth furnace 17 by using a suitable charging unit 13 to form a pellet layer on the rotary hearth 1.
(4) The pellet formed a layer is radiant heated for reduction by combustion of a burner 17c installed in the upper part of the furnace inside to produce reduced iron through metalizing.
(5) Such reduced iron is cooled to a temperature allowing mechanical handling by direct gas spraying to reduced iron by a cooler 18 or indirect cooling by using cooling water jacket, and then discharged from the furnace by a discharging unit 12 to obtain reduced iron products.
FIG. 6 is a sectional view of the rotary hearth furnace for illustrating the said process (4). The rotary hearth 1 has a number of wheels 19 attached to the bottom thereof so as to be rotated at a constant speed on a circular track 20 by a driving unit (not shown). Meanwhile, a hood 21 for covering the upper part of the rotary hearth consists of a ceiling 22 and side wall 2 and fixed to the ground. Therefore, it is necessary to shield gas within the furnace and the atmospheric air while allowing the rotary hearth 1 to be rotated freely about the hood 21, and water-sealing means 4 is installed in general between the rotary hearth 1 and the side wall 2. The water sealing means 4, as shown in FIG. 6, comprises a metallic circular box called seal trough filled with water and integrally installed at the lower end of the both side walls 2, and a downward metallic circular skirt 4a integrally installed in the hearth 1 under the both lateral ends 1a of the rotary hearth 1, in which the leading ends of the skirt 4a are immersed into water 4b in the seal troughs 4 without contacting with the seal troughs 4.
Since the hearth 1 is radiant heated up to a high temperature from the upper part by the burner 17c installed in the hood 22 when the pellets 7 are loaded on the upper part thereof, a structure is adopted in which an insulating unshaped refractory 5 is layered at the lower surface side of the hearth and a heat-resisting unshaped refractory is layered at the upper surface side of the hearth.
Further, since heating and cooling are repeated in a short time period (for 10 to 20 minutes) near the upper surface of hearth 1 for loading the pellets 7, general grade of refractory may be damaged by spalling and the like. Also, due to the various factors such as rolling, abrasion or dropping impact in supplying the pellets 7 to the upper surface of the hearth 1, fine generated from the pellets 7 is mixed into the furnace 17 together with the pellets 7 and reduced into iron powder to form an accumulated material layer on the upper surface of the hearth 1. Therefore, near the upper surface of the hearth 1 is frequently formed by a hearth material having iron oxide as the main ingredient to be free from the problem of spalling and easily remove the accumulated material layer.
Further, as shown in FIG. 6, in order to prevent the pellets 7 from falling down to the seal trough 4, the pellets 7 are not loaded near the tips of the hearth lateral ends 1a. Therefore, while the refractory upper surface 1u of the hearth lateral ends 1a is directly exposed to radiant heat from the furnace inside such as the hot burner 17c, the inside of the ceiling 22 of the hood 21 and the inside of the side wall 2 and raised in temperature to expand in a large margin, the refractory side walls 1s of the hearth lateral ends 1a are not directly exposed to radiant heat and thus do not expand. Therefore, a large amount of stress caused by heat distortion takes place in the edge 1e (hereinafter will be referred to as refractory hearth edge of the hearth lateral ends 1a, which tends to have spalling together with low strength due to insufficient burning. When the hearth edge 1e has spalling, broken pieces fall down to the seal trough 4 and the pellets 7 loaded near the hearth edge 1e tend to fall down to the seal trough 4 as well. This not only decrease the yield of the reduced iron products but also may stop the rotation of the hearth 1 since a sludge-form deposit occurs at the bottom of the seal trough 4 and the leading end of the skirt 4a is buried in the deposit.
Accordingly, it is an object of the present invention to provide a hearth structure, by which a refractory at hearth lateral ends is not damaged and carbon containing materials (such as carbon containing pellet) do not fall down to water sealing means (seal trough). Further, it is another object of the invention to provide a method for improving the yield of reduced metal.
As means for solving the said problems, the invention according to claim 1 relates to a rotary hearth furnace for producing reduced metal through heating and reducing carbon containing materials composed of metal oxide-containing material and carbonaceous reduction material, comprising: a hearth for loading said carbon containing materials thereon; and a hood for covering the whole hearth form upside, wherein the upper parts of both lateral ends of said hearth are covered with side wall lower end of the hood; and cooling means is provided at the lower end of the side wall. In this case, while being covered with the side wall lower end of said hood, the upper sides of the both lateral ends of said hearth upper surface may be covered with a portion of the side wall lower end.
The side wall lower end of the hood covers over the lateral ends of the hearth upper surface so that the refractory of the lateral ends is not directly exposed to radiant heat from the furnace inside. Then, the hearth edge is not heat distorted and spalling thereof may not take place. Also, the cooling means is installed in the side wall lower end to moderate heat distortion in furnace inside edges of the side wall lower end and to avoid spalling in these portion.
The invention according to claim 2 relates to the rotary hearth furnace for producing reduced metal according to claim 1, a vertical section including a rotary axis of said rotary hearth furnace has a combination of xcex8 and L satisfying both of the following equations 1 and 2:                                           L            ·            tan                    ⁢                      xe2x80x83                    ⁢          θ                ≧        30                            Equation  1                                L        ≧                              0.16            ⁢                          xe2x80x83                        ⁢                          θ              2                                -                      2.44            ⁢                          xe2x80x83                        ⁢            θ                    +          92                                    Equation  2            
wherein xcex8(xc2x0) is the minimum depression angle for exposing at least a portion of the hearth lateral end when said hearth lateral end is looked downward from the leading end at said hearth center side in the side wall lower end, and L (mm) is the horizontal length from said leading end to a position on said lateral end to be the minimum depression angle.
According to this invention, since the temperature of the edge can be reduced to such a degree that spalling may not take place in the hearth edge when the atmospheric temperature in the furnace is at or under 1200xc2x0 C., the effect of the invention according to claim 1 can be ensured.
The invention according to claim 3 relates to the rotary hearth furnace for producing reduced metal according to claim 2, wherein said equation 2 is replaced from Lxe2x89xa70.16xcex82xe2x88x922.44xcex8+92 to Lxe2x89xa70.19xcex82xe2x88x922.44xcex8+100.
According to this invention, even if the atmospheric temperature in the furnace is higher than 1200xc2x0 C., the temperature of the hearth edge can be reduced to such a degree that spalling may not take place in the hearth edge when the atmospheric temperature is at or under 1400xc2x0 C., and thus the effect of the invention according to claim 1 can be ensured.
The invention according to claim 4 relates to the rotary hearth furnace for producing reduced metal according to any one of claims 1 to 3, wherein said lateral end has an inclined surface with upward gradient toward a lateral leading end opposed to the hearth central side.
According to this invention, carbon containing materials do not fall down to the water sealing means (seal trough) without interfering rotation of the hearth.
The invention according to claim 5 relates to the rotary hearth furnace for producing reduced metal according to any one of claims 1 to 4, wherein said lateral end is formed of a burned precast refractory or a shaped refractory.
According to this invention, the refractory of the lateral end can be burned outside the furnace to have a sufficient degree of strength, by which the problem of low strength of the lateral end due to the insufficient burning described in the above can be prevented so that spalling of the hearth edge can be prevented more effectively.
The invention according to claim 6 relates to a method of producing reduced metal through heating and reducing carbon containing materials composed of metal oxide-containing material and carbonaceous reduction material, comprising the steps of: charging said carbon containing materials on said hearth of rotary hearth furnace for producing reduced metal according to any one of claims 1 to 5; and hot reducing the carbonaceous heat-treated products.
According to this invention, spalling does not take place in the hearth edge and thus the pellet may not fall down to the seal trough so that the yield of reduced metal can be improved.